The overall objective of this proposal is twofold, 1) to continue our investigations of oncogenic Kit receptor signaling in vivo with emphasis on hematopoiesis and 2) to develop mouse models for imatinib resistant GIST. The Kit receptor encoded at the murine W locus functions in hematopoiesis, gametogenesis, melanogenesis and gut motility. Normal Kit receptor mediated functions include cell proliferation, cell survival, cell adhesion, cell migration, secretory responses and differentiation. In human neoplasia oncogenic activation of Kit has roles in gastrointestinal stromal tumors (GIST), mastocytosis/mast cell leukemia, acute myelogenous leukemia, a subset of melanomas and a subset of germ cell tumors. Kit receptor functions are mediated by kinase activation, receptor autophosphorylation and association with various signaling molecules and signaling cascades. How do receptor tyrosine kinases such as Kit mediate distinct cellular responses in different cell types during embryonic development and in the postnatal animal? And what are the requirements for oncogenic transformation in different cell types to produce cancer. We have produced mice containing knock-in point mutations in the Kit receptor gene in mice which block Kit mediated PI 3-kinase activation and signaling or Src family kinase, SFK, activation and signaling. These mice have distinctly different phenotypes in gametogenesis and hematopoiesis. We have also investigated the role of PI 3-kinase and Src family kinase signaling in Kit mediated cell proliferation, suppression of apoptosis, cell adhesion, chemotaxis and secretory responses in vitro in bone marrow derived mast cells (BMMC). Whereas Kit mediated PI 3-kinase signaling is critical for cell proliferation, cell survival, cell adhesion, chemotaxis and secretory responses, Kit mediated SFK signaling has mostly a negative regulatory role. Furthermore, gene expression profiling indicates that blocking SFK signaling reduces KitL induced expression of TH2-cytokine genes in BMMC. To investigate the role of oncogenic Kit activation Kit in tumorigenesis we have produced mice carrying a juxtamembrane domain knock-in Kit mutation and reproduced human familial GIST syndrome (Sommer et al., 2003). In part our proposed studies will attempt to investigate the role of PI 3-kinase, SFK and Ras signaling in Kit-mediated tumorigenesis. Furthermore we will produce and characterize mice carrying oncogenic imatinib resistant Kit alleles.